1. Field of the Invention
The present invention relates to a discharge device, an image formation unit, and an image forming apparatus, such as a copier, a fax, a printer, etc., employed in the discharge device.
2. Discussion of the Background Arts;
In the image forming apparatus, such as an electrophotograph apparatus, an electrostatic recording apparatus, etc., a surface of a latent image carrier, such as a photoconductive member, a dielectrics, etc., is discharged to form a latent image. As a discharge device, a scorotron system, which applies high voltage to a tungsten wire and creates and executes corona discharge or a corotron system is employed. However, such a system needs a large power and creates a problem in that a significant amount of ozone or oxides of nitrogen (NOx) is output into atmosphere due to corona discharge created by the high voltage application.
In these years, various contact type discharge devices have been proposed such that a discharge member contacts an image bearer without executing corona discharge as discussed in Japanese Patent Application Laid Open No. 63-7380. Since a low voltage is applied to a discharge roller in this discharge system, an amount of ozone is advantageously small extraordinarily. Thus, such a system is increasingly employed in a copier and a printer, recently. This contact typed is charge device also resolves many other problems caused by the above-mentioned discharge device executing corona discharge.
Such a contact type roller discharge system uses a rubber or plastic discharge roller to stably contact a photoconductive member. However, the photoconductive member is polluted due to emergence of low-molecular component included in the rubber or plastic as a problem. Also created as a problem is traversal lines appearing at a pitch of the photoconductive member or the a discharge roller. Further, since the discharge roller contact the photoconductive member, toner or paper dust and equivalent remaining on the photoconductive member sticks and pollutes the discharge roller. Further, an abnormal image, such as a ghost image, etc., is created, and an amount of abrasion of an image bearer increases.
Then, it has been recently proposed that a small gap is created between a discharge member and an image bearer to suppress a problem of quick deterioration of the surface of the image bearer as discussed in Japanese Patent Application Laid Open No. 2002-55508.
Further, in order to resolve a problem, in which a discharge voltage changes and accordingly an abnormal image is created when a gap length changes, and stably execute discharge, a fine unevenness is formed on a surface of a discharge device as discussed in Japanese Patent Application Laid Open No. 7-287433. Specifically, uniform discharge is executed while avoiding creation of the abnormal image even though the gap length changes. However, since such a fine unevenness varies as time elapses or sometime disappears, when exposed to a severe condition, such as discharge, etc. When such unevenness disappears, an originally intended goal can't be reached, and accordingly, a uniform discharge can be hardly executed for a long term. Further, since fine unevenness is limited to a prescribed range, a production process becomes difficult and costly.
Further, as a discharge bias applying system, a DC voltage applying system, and a DC/AC super imposed voltage applying system (hereinafter referred to as a superimposed DC+AC system) are exemplified. The DC voltage apply system is hardly used practically due to unevenness of discharge voltage caused by gap length variation and a problem of instability of discharge. It is known that the DC/AC superimposed voltage applying system is more preferable than the DC voltage apply system as to stability of discharge voltage during gap length variation. Thus, the DC/AC superimposed voltage applying system is believed to be suitable system for non-contact type discharging.
It is also known that an AC voltage applying system largely receives damage on a photoconductive member. Further, discharge generates a large number of discharge products in a discharge section as many as corona charge. That is, the DC/AC superimposed type discharges an enormous number of times more than the DC discharge system. Further, the DC/AC superimposed type creates a problem of noises when alternate current is used for an application voltage.
In the DC voltage applying system, it has been proposed that a plurality of members is provided to contact a photoconductive member to suppress variation and stabilize the gap. However, there is a limit to suppression of a size error when a discharge roller and a photoconductive member are manufactured. In addition, parts cost increases.
As mentioned above, a difference in a discharge voltage takes place due to variation in length of a gap between a discharge roller and a photoconductive member when the non-contact type DC discharge system is employed. When the voltage difference causes a difference in toner adhesion amount when an image is visualized in a developing process, unevenness takes place in an image. Further, gap length unevenness periodically occurs due to eccentricity of the discharge roller or the photoconductive member, thereby causing image unevenness. To suppress such eccentricity, a size precision is strictly demanded in a discharge roller or a photoconductive member, thereby increasing a parts cost.